GBT S-band RFI survey, part 2: Feb 7-8, 2002

by F. Ghigo
(Feb.12, 2002)

Summary

On Feburary 7-8, 2002, a survey for interference
was done with the GBT 2_3 GHz receiver over the
frequency range 1.7 to 2.6 GHz using the Spectral Processor.
Data were acquired with the GBT positioned at four different
azimuths (az = 6, 96, 186, and 276 degrees), all at an elevation
of 55 degrees. Linear polarization was used.
The laser power and computer systems
were off for these measurements. The
results are compared with data from the previous week in
which the laser systems were on.

Several RFI signals appear on this recent data that did
not happen in the previous week, when the laser systems
were on. Signals at some of the same frequencies as the
previous week appear at drastically different power levels.
In view of the apparent variable nature of many
of the sources of RFI, we cannot definitely associate any
signals with the laser systems. A better approach to
identifying RFI from the laser systems might be to switch
the lasers on or off every hour, doing a spectrum sweep
in between. Another approach would be for the
IPG to track down the various RFI peaks that have been
identified in this survey.

The major RFI features are the same as in the previous
survey. Refer to the previous writeup for plots.
The directional nature of the signals is also similar.

To compare the results from the Jan 30 data with the present
data, a plot of the difference in the filtered data is
shown in
Figure 4
and Figure 5. See further discussion
below in the results section.

Observations

The observing used the same setup and sequence of
scans at successive sky frequencies as in the previous
run (Jan 30-31). The journal of observations is
summarized in Table 1. All observations were done with
the GBT at elevation 55 degrees.

Table 1: Observations 08 Feb 2002,
project: rfi_S_feb07

Scan #s

Start Time (UT)

Azimuth (deg)

21-48

02:24

96

49-77

03:30

186

78-105

04:44

276

106-133

05:49

6

134-161

07:01

96

162-189

08:06

186

190-217

09:12

276

218-245

10:18

6

246-273

11:32

96

Processing

Results

Raw Spectra

The spectra averaged over all 4 azimuths is shown in
Figure 1. The last spectral sweep (scans 246-273) was not included in
this average, in order to exactly match the average that
was done in the previous run, i.e., 8 spectral sweeps with
2 sweeps at each azimuth.

The spectrum is again dominated by the satellite RFI in the range
2320 to 2345 MHz. Weaker features can be seen in
Figure 2,
plotted with an expanded scale.

Again, we note that the noise level is much
higher in X polarization (red plot) that in Y (green) at frequencies
where the strong satellite RFI is in or near the band.

Filtered Spectra

The filtered, baseline-flattened spectra were produced as described
before by dividing each spectrum by the median-filtered version of
itself to brings out narrow band features.

Figure 3 shows the filtered
averaged over all scans: the X polarization is shown in red,
Y in green.
Again, the spectrum is cleaner for
frequencies less than 2200 MHz in the Y polarization.

The list of RFI peaks based on the data plotted in Figure 3
is given in Table 2.
All peaks exceeding 1.002 are listed in Table 2, omitting those
between frequencies of 2485-2500 and 2315-2347 MHz.
The power (P) in Table 2 is the y-coordinate
transformed to 0.001 units above 1.0;
i.e., P = 1000(Y-1). i.e., the power is in parts per thousand of
the system temperature. The maximum of the 2 polarizations is listed.

Note particularly strong RFI around 2460-2470 MHz, and at 2200 MHz,
that was not very notable in the Jan 30-31 data.
Close-up plots of these features in the raw data are
shown in
Figure 6A
and Figure 6B.

Laser dependant RFI?

To look for differences between the two observing runs, we present
the difference between averaged filtered spectra for the Jan 30-31
run and the Feb 7-8 run.
Figure 4 shows this "on-off"
spectrum, and
Figure 5 shows the same
data on an expanded scale.
In these plots, the positive-going spikes are signals that were
stronger in the Jan 30-31 data, i.e., with the laser systems on.
Likewise, the negative-going spikes are the signals that are
stronger in the Feb 7-8 data, i.e., with the laser systems off.

Inspection of these plots suggests there was more and stronger
RFI on the latter date than the former.

We have listed tables of RFI components from the difference spectra.
Table 3 are the components that
were stronger on Jan 30-31, and
Table 4 lists those stronger on Feb 7-8.
Amplitudes are listed in units of 0.001 of the system temp.
The tables list all components exceeding 0.002 of the system
temp.

Again, most of the RFI seems associated with the az=96 spectrum,
i.e., in the general direction of the Jansky Lab. In particular
the very strong features at 2200 MHz and 2467 MHz seem associated
with this direction.